Current loop

A current interface or current loop interface works with a power adjustment ( however, standard is a voltage adjustment). It is used for information transmission in harsh environments where high noise levels can be expected, so that the usual means of transmission is not operating voltage levels with sufficient reliability.

Analog phones in industrial applications as well as electronic musical instruments (MIDI) work preferably by means of an impressed electric current. Striking were also Telegraph.

In the simplest case, the transmitter or transmitters is in the current interface and an adjustable current source of the receiver consists of a low impedance terminating resistor ( shunt resistor ) is determined via the voltage drop on the measuring current.

Analog current interface

Become an analog voltage values ​​from a transmitter via a cable to a receiver, they will be distorted. Cause the necessary measuring current produces a voltage drop in the cable due to undesired cable internal resistance. By high-impedance as possible inputs to the measured current can be minimized and thus the measurement error. What remains is due to the high impedance structure, a high sensitivity to disturbances capacitive and inductive Art

Therefore measurements over longer distances are preferred as a current transfer (4 to 20 mA). In the power transmission of the internal resistance and thus the voltage drop of the cable play almost no role. Furthermore, the power of the measured value most sufficiently large compared to the capacitive coupling of noise currents ( e.g. 50 Hz hum). Induction noise can be minimized relatively easily, by a twisted cable.

• With a current of at least 4 mA allows the transmitter supply (only one return line required).
• Cable break ( current < 4 mA) can be detected.

Digital current interface

In digital transmission, the presence or absence of a stream is interpreted as binary information takes a different voltage level at the current interface.

The 20 - mA current loop interface is standardized in accordance with DIN 66258. She works with a maximum open circuit voltage level of 24 V. Since this voltage leads steep signal edges to considerable crosstalk, the transfer of 20 mA interfaces is not allowed in telecommunication cables.